AnAnthropologicalPerspective:AnotherDimensionto … · 2012. 8. 24. · evolution, diet, environment, form, ... In addition to the anthropological approach, paleontological studies

Hindawi Publishing CorporationInternational Journal of DentistryVolume 2012, Article ID 741405, 6 pagesdoi:10.1155/2012/741405

Review Article

An Anthropological Perspective: Another Dimension toModern Dental Wear Concepts

John A. Kaidonis, Sarbin Ranjitkar, Dimitra Lekkas, and Grant C. Townsend

School of Dentistry, The University of Adelaide, Adelaide, SA 5005, Australia

Correspondence should be addressed to John A. Kaidonis, [email protected]

Received 24 August 2012; Accepted 28 October 2012

Academic Editor: Anne-Marie Grimoud

Copyright © 2012 John A. Kaidonis et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

For many years, research on tooth wear by dental academics has been diametrically opposite to that of anthropological research,with each discipline having a different understanding as to the nature of the wear processes. Dental focus revolved aroundpreventive and restorative considerations while the anthropological focus was a biological understanding related to humanevolution, diet, environment, form, and function and included all the craniofacial structures. Introducing the anthropologicalperspective into modern dentistry gives an insight into the “bigger picture” of the nature and extent of tooth wear. By combininganthropological evidence with clinical knowledge and experience, it is most likely to provide the best-informed and biologicallybased approach to the management of tooth wear in modern societies.

1. Introduction

The current mechanisms of tooth wear that are generallyaccepted within the dental literature include abrasion, ero-sion, and attrition. Abrasion occurs from the friction of anyforeign substance forced over tooth surfaces (e.g., tooth-brush, food); erosion is a modern-day condition resultingfrom the dissolution of tooth substance by acids that donot originate from oral biofilms, and attrition results fromtooth grinding without the presence of food. Although it isnot the purpose of this paper to detail the specifics of eachmechanism, it can be said that abrasion caused by food gen-erally acts more broadly on a tooth surface eventually causingdentinal scooping, whereas the characteristic feature ofattrition is the production of well-defined facets on opposingteeth. Erosion can act on nonoccluding surfaces, does causedentinal scooping, and can be distinguished clinically by thecharacteristic appearance of a glazed affected surface [1]. Themicrowear detail resulting from these mechanisms enablesthem to be distinguished from one another reasonably easily,particularly if one of the mechanisms is dominant. However,they often act together with different intensities and dura-tions, making it difficult to differentiate between them [2].

In addition to the mechanisms described above, abfrac-tion can also be included as a mechanism of tooth loss.Although currently controversial, it nevertheless may beconsidered as an added factor contributing to noncariouscervical lesions [3–6]. Erosion and abfraction are certainlymodern-day conditions, while abrasion and attrition havebeen described in different species and phyla throughout thecourse of vertebrate evolution.

Human tooth wear in precontemporary populations hasbeen studied by physical anthropologists before the lastcentury. Efforts to find associations between tooth wearand culture, gender, age, craniofacial geometry, tooth size,dental crown traits, diet and environment, and so forth,have provided many new insights over the years [7–9]. Incontrast, there is relatively little information about the extentand nature of genetic and environmental contributions tovariation in tooth grinding [10].

Anthropological understanding of tooth wear in pre-contemporary populations was focused, mainly, for manyyears, on the abrasiveness of food, cultural practices, and theuse of teeth as tools [7]. The mechanisms of attrition anderosion, according to today’s definitions, were not consideredin any detail. Unfortunately, the terms were often used

2 International Journal of Dentistry

synonymously by anthropologists, referring to wear causedby an abrasive diet, thereby adding some confusion in theliterature.

Furthermore, anthropologists considered tooth wear as anormal physiological process, with the teeth in most indi-viduals remaining functional throughout life. The changesto tooth surfaces and to the dental occlusion resultedin a series of cascading adaptive processes involving thewhole stomatognathic system [11]. It was only when theteeth became so worn that they were nonfunctional, that apathological state was considered to be present.

In contrast to the anthropologist’s perspective, toothwear research from a dentist’s viewpoint has tradition-ally focused on restorative management and it has onlybecome a major topic of interest among dentists duringthe last 20 years or so. Much of the research in this areanowadays concentrates on improving understanding andmanagement of modern-day conditions (e.g., erosion). Somedental researchers, especially those who trained accordingto “gnathological” principles, have maintained a morerestricted mechanical view of tooth wear and considered anyevidence of wear to be pathological [12]. This dichotomybetween anthropological and dental research reflects the ori-gins and nature of each discipline; however, both disciplinesshould be viewed as components of a “bigger picture.”

Contemporary populations show little abrasive wearbecause of the consumption of processed, softer food.Therefore, much of the wear in our modern societies resultsfrom erosion and attrition. While attrition is common incurrent populations, there is evidence that attrition caused bytooth grinding may have always been a common behaviourin humans [13].

In summary, without an awareness and understanding ofthe considerable amount of anthropological research that hasbeen carried out relating to tooth wear, many dentists retaina general impression that today’s mostly unworn dentitionsrepresent the “norm,” when in fact the post-industrialhuman dentition and occlusion tends to represent a neote-nous stage of development. That is, it tends to present in asimilar manner to the relatively unworn dentition of a youngPaleolithic individual. In addition to the anthropologicalapproach, paleontological studies associating dental wearwith dental and craniofacial anatomy, the evolution of cusps,form and function, and animal behavior also add insightsto the “bigger picture” of the nature and extent of toothwear.

2. Precontemporary Populations

There are many “interesting” dental features that have beendescribed and documented by anthropologists that expressthe true dynamic nature of the craniofacial skeleton anddental occlusion. Some of these features will be brieflydescribed to indicate the nature and extent of variation thatcan be observed in the stomatognathic system from bothspatial and temporal perspectives. Many dental practitionersare unaware of these features and they are generally notconsidered in any depth, if at all, in dental curricula.

2.1. Interproximal Wear. Interproximal wear occurs whenthe proximal surfaces of teeth rub together. The differentialloads acting on teeth cause them to move independentlyfrom one another during function. The resulting interprox-imal wear is proportional to the observed occlusal wearand therefore the load subjected on the teeth [14]. Whenteeth erupt, early interproximal contact points eventuallybecome contact areas and the contacts between adjacentteeth remain closed due to mesial drift. This results in areduction in dental arch length, the amount of which isproportional to the degree of wear [15–17]. This observa-tion also underpinned the theory of attritional occlusion[16], which proposed that interproximal wear reduced theprevalence of dental crowding and third molar impactionsas more room was created within the dental arch. Thistheory, when extrapolated to modern orthodontics, wasalso partly used as a rationale for the extraction of pre-molar teeth during orthodontic management of crowdeddentitions.

Commonly, with interproximal wear, the mesial surfacesof teeth wear faster than the distal surfaces of adjacentteeth resulting in the development of mesial concavities(Figure 1). These tend to be more evident in dentitionsexhibiting advanced wear [14]. Explanations for this char-acteristic wear pattern have been proposed to involve thedistribution of force vectors through the teeth under load[18]. Although this type of wear is often considered to beinsignificant by dental practitioners, there is strong evidencethat dental occlusions do change over time through adynamic process that anthropologists consider to be quitephysiological.

2.2. Interproximal Grooving. Interproximal grooving canbe considered to be a type of noncarious interproximallesion most often seen on the distal surfaces of posteriorteeth in hunter-gatherer populations, particularly AustralianAborigines (Figure 2). This type of abrasive wear results fromcultural practices where the teeth are used as tools. For exam-ple, at times when kangaroo tendon was chewed and passedbetween the teeth, it could slip interproximally and wear thedistal surface of a tooth as the tendon was pulled anteriorly[19].

2.3. X-Occlusion. “X-occlusion” or “alternate intercuspa-tion” are two terms used to define a characteristic modeof occlusion observed among Australian Aborigines livingin their traditional lifestyle (Figure 3). Here, the dentalarches exhibit two different centric occlusions resultingfrom the upper arch being wider than the lower. Whenthere is maximum intercuspation on the left side, theright side displays a large overjet, and vice versa [20].The dentitions of these individuals are fully functionalyet, according to “modern” dental opinion, such dentitionswould generally be considered nonfunctional and orthodon-tic treatment would likely be recommended. This is agood example of how opinion about dental issues is oftendependent on the breadth and depth of one’s educationalbackground.

International Journal of Dentistry 3

Figure 1: Interproximal wear in a young child. The mesial of thepermanent first lower molar wears faster than the distal of theprimary second molar.

Figure 2: The passing of kangaroo tendon between posterior teethcaused distal abrasive wear called “interproximal grooving.”

2.4. Helicoidal Plane. This occlusal characteristic has beendocumented by anthropologists in different human popula-tions and has been described as a posterior occlusal twist ofworn occlusal surfaces [21, 22]. Here, the occlusal surfaces ofthe lower first molars slope towards the buccal, the secondmolars are horizontal, and the third molars slope lingually(Figure 4). The opposing teeth follow the reverse pattern.This alteration in the occlusal curvatures can also occur inmodern societies where there is advanced dental wear, butthese changes often go unnoticed.

2.5. Form and Function, Differential Wear, and Occlusion.Contrary to belief within some dental circles, dental occlu-sions are dynamic and continually changing. Tooth wear,particularly abrasion in precontemporary hunter-gathererpopulations, is the main mechanism that changes the mor-phology of teeth over time. A “canine-protected” occlusionduring youth tends to change progressively into “groupfunction,” causing a number of consequential adaptivechanges [23]. As the wear progresses, continual eruption ofteeth compensates for the wear (a feature seen commonly inother nonhuman species as well). The established occlusalvertical dimension at any given point of time is, therefore,a by-product of these two opposing processes. As the cuspsreduce in height and disappear, the “tear-drop” cycle of themasticatory stroke becomes wider and, with excessive wear,

Figure 3: An example of “X-occlusion” or “alternate intercuspa-tion.” The plaster models sectioned coronally across the molarshighlight the two types of centric occlusions.

Figure 4: The helicoidal plane. The wear pattern produces anocclusal twist of the molar teeth. The buccal inclination on the firstmolars changes to a neutral inclination on the second molars thatin turn changes to a lingual inclination on the third molars.

corresponding remodeling of the glenoid fossa also tends tooccur [24].

Exposure and scooping of the dentine by abrasion(leading to differential wear) is important in maintainingphysiological function, often described as a scissorial action,supported by a wider “tear-drop” shape of the horizontalaspect of the masticatory stroke. Scissorial point cutting isa common evolutionary feature across different species andphyla. This feature has been described in the literature, isuniversal among herbivores, and is also strongly evident inhuman and nonhuman primates [2, 25].

The scooping of dentine allows the resulting, less-wornenamel to act as a sickle-shaped blade that moves against asimilar blade facing in the opposite direction in the opposingarch. An extension to this well-documented account, whichexplains how teeth not only grind but actually cut food,is the theory of thegosis. This theory was developed over50 years ago by Ron Every, whose observations of differentspecies (including humans) identified that tooth grinding


was a sharpening mechanism of the sickle blades resultingfrom differential wear to enhance the masticatory efficiency[26]. In addition, Every extended his observations to spe-cialisations within other species (e.g., baboons, wild boars),and showed how tooth grinding occurred during stress-ful encounters (fight-or-flight response) often in order tosharpen the canines—their biological weapons. This theorywas further extended to the notion that tooth grinding inhumans is built into our genetic codes, and is, therefore, aninstinctive universal behavior [25, 27–30].

Studies involving differential wear seem to indicate thatdentine exposed by abrasive action is not usually sensitivebecause a mechanical smear layer forms on the tooth surface,thereby occluding dentinal tubules. When dentine is foundto be sensitive, there are open dentinal tubules causedby superimposed erosion. In addition, studies on width-versus-depth ratios of scooped dentine show that abrasivescooping tends to be shallower than that caused by erosion[31].

Tooth wear mechanisms, such as attrition and in par-ticular abrasion, have been present since the reptilian-mammalian transition. These mechanisms were a selectiveforce that was responsible for many adaptive evolutionarychanges, as well as masticatory specialisations, specific foreach species.

There is an acknowledged common pattern of wearobserved between opposing posterior teeth in most species.The buccal cusps of the lower molars wear faster thanthe lingual cusps, while the palatal cusps of the uppermolars wear faster than the buccal cusps. Dentists andanthropologists agree that a “tear-drop” masticatory action,together with a power-stroke, can explain this pattern ofwear. Interestingly, accessory cusps are seen on the palatalsurfaces of the upper teeth (e.g., Carabelli trait) and onthe buccal cusp of the lower molars (protostylids) [32].These accessory cusps are able to “take over” the load asthe working cusps wear. This example shows how toothwear, from an evolutionary perspective, is a selective forcethat is likely to have been responsible for changes in dentalmorphology.

Differences in the extent and pattern of tooth wear haveoften been observed between the dentitions of precontempo-rary populations. These relate commonly to environmentaldifferences in diet; however, social and cultural determinantscan also contribute to these differences. For example, thereare recorded differences in wear patterns between males andfemales in some precontemporary populations because ofdifferences in the tasks undertaken by each sex [33]. Somesocieties even “file down” anterior teeth for aesthetic reasonsor have teeth avulsed as a cultural practice [34].

3. Contemporary Populations

So-called contemporary, modern or post-industrial popu-lations tend to show wear characteristics that are differentto those of our ancestors. As a general rule, the wearcaused by abrasive food is much reduced in modern societiesbecause of our softer, more processed diets. However, there

Figure 5: The “vertical” positioning and crushing of pumpkinand watermelon seeds using the anterior teeth has resulted in theabrasion “notches” in those teeth.

Oblique

Transverse

Figure 6: An impression of an occluding surface of an excessivelyworn night guard. The transverse and oblique wear patternsindicate the lateral mandibular movement that formed them. Inclinical and micrographic assessments, the wear pattern would beevident as striations.

are still dietary differences between current populationswhere the preparation and consumption of specific foodscan result in nontypical abrasion patterns. Figure 5 displaysa specific abrasive wear pattern caused by the crushingof dried pumpkin and watermelon seeds between theincisors.

3.1. Attrition. Microwear patterns within facets character-istically have parallel striations that are orientated in ahorizontal or oblique direction on posterior teeth and in ananterior-lateral direction on anterior teeth. This microwearpatterning has been described in the past leading to twodiametrically opposing conclusions. One conclusion statesthat the microwear patterns have resulted from mandibularaction during mastication [35], while the other conclusionstates that tooth grinding is the cause of these patterns [27].Recent studies indicate that tooth grinding is indeed the mostlikely mechanism involved with these patterns, as we haveobserved the same patterns on the occlusal surfaces of nightguards (Figure 6). This view is also supported by case studiesthat show wear patterns resulting from eccentric mandibularmovements (Figure 7).

International Journal of Dentistry 5

Figure 7: An extreme lateral mandibular movement past the canineedge-to-edge has resulted in the notch on the upper right centralincisor.

The mandibular movement during grinding results froma lateral movement where the lower canine tends to ridealong the palatal surface of the upper canine and pastthe canine-edge-to-edge position into an eccentric position.Here, one condyle pivots at the postglenoid tubercle (ipsi-lateral), while the contralateral condyle moves anteriorly,medially and inferiorly along the condylar eminence dueto the unilateral action of the lateral pterygoid muscle. Theresulting microwear striations are generally seen on the wearfacets of anterior teeth, but they also become evident on theposterior teeth where the group function is present. Thesestriations have been described as transverse and oblique onposterior teeth and anterolateral on anterior teeth [27].

Often, unusual wear patterns can be observed in dentalsurgeries where anterior teeth have worn much faster thanposterior teeth. Although further research is required totease out the reasons for these differences, it appears thata patient’s craniofacial morphology, together with theirocclusal scheme, plays a role. For example, an Angle Class2, Division 2 (or a deep anterior bite) malocclusion tendsto be associated with excessive anterior tooth wear. Thesimple geometry of having a deep bite implies that a canine-protected occlusion will continue for a longer period of time,thereby exacerbating the wear on the anterior teeth.

3.2. Social, Cultural, Iatrogenic, and Other Determinants. Asin past populations, modern societies are also influenced byprevailing fashions, often with severe consequences. Tongueand lip piercing fall into this category, commonly causingmechanical damage to adjacent teeth.

Similarly, the dental bur has been responsible for iatro-genic dental wear when occlusal equilibrations have beenperformed in dental practices. Although there are differencesof opinion about the need for occlusal equilibrations andthe resulting wear has been considered by some operators tobe relatively minor, these procedures, nevertheless, removetooth structure irreversibly.

There are occasions when tooth wear patterns areobserved without any obvious explanation, even after thor-ough questioning of the patient. An example is depicted in

Figure 8: Two wedge-shaped lesions have formed on the lower firstcentral incisors. To date there is no definite explanation as to thecause of the wear pattern.

Figure 8 where wedge-shaped lesions were observed on theworn incisal edges of lower central incisors.

4. Conclusion

Tooth wear patterns vary within and between human popu-lations, both past and present, for a variety of reasons. Theunderstanding and opinions formed about the nature andextent of tooth wear within the human dentition still tendto reflect the training and educational background of theobserver. In our view, combining anthropological evidencewith clinical knowledge and experience is most likely toprovide the best-informed and biologically based approachto the management of tooth wear in modern societies. Theanthropological evidence introduces a broader view of thepattern and extent of tooth wear within and between popu-lations, allowing for a “rethink” of some of our current dentalconcepts.

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AnAnthropologicalPerspective:AnotherDimensionto … · 2012. 8. 24. · evolution, diet, environment, form, ... In addition to the anthropological approach, paleontological studies